Rose, first welcome to Cruiseline.com. I did a similar cruise a few years ago from San Juan Porto Rico to Southampton on RCI Adventure Of The Sea's. You should have a great time. We had 3 formal nights on that 14 night cruise. I did not bother with my Tux and just took a dark suite. 2/3 of the ship were Brits and the ladies did dress a bit more then the Americans. It is a hard trip to pack for because you start out in Warm weather and end up in cold and rainy weather (well, cold to me, I'm a native of Los Angeles). I recommend you packing so you can layer, a pair of shoes for rain, and a water proof coat.
Remember the Pound Sterling is worth more then the USD, if your staying in the UK things will be more expensive. Today the rate is 1 GBP to 1.42 USD.
If your going to stay a bit in London I suggest
1. On departure day the buss to the airport was $ 100.00, the Excursion to Stonehenge was only $130.00 and dropped you at the airport, so seeing Stonehenge was really only 30.00.
2. From Heathrow Airport you can get the tube (Subway) into London. I think in the basement of terminal 4 or 5 has the sub way stop (ask your bus driver). You want the Piccadilly line to go to London without having to transfer.
3. If your staying in London and using the sub way I suggest buying a weekly pass called an oyster card. Much less expensive then paying for each trip.
4. I used a service for hotel and information called London Town that I had found on the internet that recommended a hotel in a good area that was in our budget and just 2 blocks off the tube station. They also had a book on London for us on check in. We stayed in Kensington, nice safe area to home base out of for London.
5. The food in UK is not what were used to here in the states, Pub food was good (fish and chips, bangers and mash), Indian food was good as was the Middle Eastern.
6. A lot of the Museums in London are free to get into and worth an afternoon to look through. Hyde park is also nice to see including the memorial to Princess Diana.
Be safe and have a great trip.